In the related art, a lamp which is representatively a halogen lamp and generates infrared rays, or a method of performing heating with Joule's heating by using electromagnetic induction is put into practical use as a heat source of a fixing device which is mounted in an image forming apparatus. In this heating method, a time for warming up the entirety of a fixing device and a lot of electrical energy are required. There is a problem that heat which is generated in the fixing device is transferred to other units of an image forming apparatus, and thus malfunction occurs.
Recently, a reduction of time taken to start the device, energy saving, prevention of excessive heat generation, and the like also become critical issues. Accordingly, a method as follows is proposed. Two heat-generating resistors having resistance values different from each other are provided in a fixing device. The heat-generating resistors are respectively connected to power supply systems which are different from each other, and thus one heat-generating resistor is constantly conducted and the fixing device is pre-heated during a time when the fixing device is on standby. With such a structure, good fixation characteristics of allowing an optimal temperature gradient in a fixing nip to be realized corresponding to various sizes of recording paper are obtained.
However, in the above-described structure of the device in the related art, when small-sized paper and large-sized paper are mixed and supplied, it is difficult to delicately control power, which is required to be supplied to a heater, to be minimized at both end portions of the heater and at the center portion.
An example of the related art includes JP-A-2000-243537.